Lemma 1. If function \(\displaystyle f\) : \(\displaystyle {R^d} \rightarrow R\)  is convex, \(\displaystyle \forall x,y \in {R^d}\) , then[1]}:
\(f(y) \ge f(x) + \left\langle {\nabla f(x),y - x} \right\rangle .\) 
